Already known in the prior art, there is German patent no 4101630 (U.S. Pat. No. 5,151,246) which describes a method for the production of porous semi-finished products from aluminum and copper-based alloy powders. The method described therein comprises the steps of mixing of an alloy powder with a foaming agent, filling a press container with the mixture, simultaneously heating the filled container and applying pressure at which the foaming agent does not decompose, simultaneously cooling and removing the pressure, disassembling of the container followed by pushing of the solid briquette out of it, which is immediately heat treated to produce a porous body or is subjected to preliminary hot deformation via extrusion or rolling followed by heat treatment. A very narrow range of products in terms of sizes and shapes can be produced with such method since the weight of the briquette is 2–5 kg. In addition, this method demonstrates a very low output because of the prolonged heating of the large size press container filled with the powder mixture. Even in the case where the powder mixture would be heated in a container having 100 mm in diameter and 400 mm in height, the heating operation would be economically not feasible.
Also known, there is a method for the production of porous semi-finished products from metallic powders that incorporates different variants.
A first variant includes the steps of coating the bottom floor of a press container with a metallic layer free of foaming agent, covering the metallic layer with a powder mixture comprising a foaming agent, and then covering the layer of powder mixture with a second metallic layer free of foaming agent. The container is then heated, and hot compaction is carried out. The shape of the body produced can be changed via deformation. Then, the body is foamed for formation of a new body wherein a high porous foamed metallic layer appears between two metallic layers.
A second variant includes the steps of disposing a dense metallic disk in an empty press container adapted for extrusion and filling the container with a powder mixture containing a foaming agent. Then, the container with the powder mixture is subjected to heating followed by the application of a pressure of about 60 MPa. Due to the pressure, the central part of the hard metallic disk, which blocks the hole of the press die, begins flowing through this hole and ensures extrusion process. During subsequent extrusion stages, the compacted powder mixture plastically deforms and flows through the die hole. Also in this case, the dense metallic layer covers the extruded powder mixture, which is ready for foaming. After foaming of this combined body the metallic layer covers a core consisting of high porous foam.
The combined billets produced via both variants can be further rolled in sheets, and due to a heat treatment temperature, can be transformed in a porous metallic body (U.S. Pat. No. 5,151,246, September, 1992, B 22 F 3/18, B 22 F 3/24).
Also known in the prior art, there is a process including the steps of mixing of an alloy powder with a foaming agent and rolling the mixture at high temperature (in the range of about 400° C. for aluminum) in several rolling passes. Intermediate heating of the pre-rolled material following the individual roll passes is a significant measure to largely avoid creation of edge cracks. This produces a bonding of metal and propellant powder particles in the roller nip and forms a gas-tight seal for the gas particles of the propellant. This body can be transformed by heat treatment in a porous metallic body.
The disadvantages of these techniques are the limited possibility of production of semi-finished products, especially sheets of commercial sizes, low product yield and output, high manufacturing cost.
Also known in the general field of powder metallurgy, there are the processes described in EP 0127312 and in U.S. Pat. No. 4,820,141 (EP0271095). These documents which do not concern the production of foam metal are given as examples of art related to the present invention.
EP 0127312 discloses a process for the consolidation of metal powders into slab configuration in which the metal powder is encapsulated, heated and inserted in a containment die and is subjected to a rolling operation to consolidate the powder.
U.S. Pat. No. 4,820,141 discloses a method for forming non-equilibrium and/or metastable metallic or non-metallic powder, foil or fine wire material into solid body. The method disclosed comprises charging the material into a metal container, subjecting the metal container containing the material to rolling at a temperature at which the inherent properties of the material are maintained, and thereafter removing the metal container.
The method of the present invention is distinct from and overcomes several disadvantages of the prior art, as will be discussed in detail below.